There are a variety of types and sizes of fuses which are presently employed in different electrical and electronic circuits, and, indeed, their use in such circuits has been known for years. As it is well known, a fuse is a device intended to melt and open an electrical circuit whenever the ampere load on the circuit exceeds a predetermined safe value, i.e., the rated capacity of the fuse. However, in some circuits such as, for example in A-C motor circuits, the fuses open too quickly on moderate overloads. In order to overcome this difficulty, so-called time-delay (time-lag) fuses are employed which open the circuit only after an overload period of several times as long as that of an ordinary fuse.
Several types of time-delay fuses are now in use. One type, for example, known as the spring type fuse, comprises a fusible wire element held taught by a spring and soldered at both ends by means of a low melting point solder element. However, since relatively thin wire is taughtly held by the spring which exerts a tensile force upon the wire, it is usually weak against mechanical vibrations, and exhibits inferior shock resistance and other mechanical properties. Additionally, the low melting point of the solder, difficulty of maintaining adequate quantities of solder and special processing techniques needed to make them lead to great difficulties in mass producing this type of fuse at low manufacturing cost and with good mechanical stability.
Other types of time-delay fuses include a fuse which has a ceramic core wound by a fusible element and designed to interrupt so-called "arcing" in the fuse by absorbing the heat generated therein. Also, a fuse having a glass fiber wound by a fusible element is employed in order to interrupt arcing in the fuse by causing the glass fiber to melt simultaneously with the fusible element. However, all of these fuses exhibit inferior time delay characteristics and they are not entirely satisfactory in some circuits.